In several contexts, it is desirable to couple the optical energy from several, such as two, semiconductor laser diodes into a single optical fiber.
This combining capability is relevant to wavelength division multiplexing (WDM) applications where the laser diodes operate at different wavelengths and are modulated in response to different information signals, but couple into the same fiber.
The combining capability is also relevant in pump or other applications where power is the primary metric. In pump applications, the light from laser diodes is used to optically-pump rare-earth doped fiber or alternatively regular fiber, in a Raman pumping scheme. Multi-laser pump modules are attractive because there are limitations in the power that can be produced by a single high-power laser diode. As electrical current is increased in these lasers, the typical failure mode is catastrophic optical damage (COD) at the facet, especially in shorter wavelength laser devices such as 980 nanometer (nm) pump lasers, or excessive current densities in the ridge. Using multiple laser modules enables powers that are greater than could be generated by a single laser module.
Despite advantages, there are few commercial, integrated examples of laser systems that attempt to couple the optical energy from multiple lasers into a fiber, especially the single mode fiber that is used in most optical communication systems today. The explanation for this is the cost to manufacturexe2x80x941) combining the optical energy from two lasers with bulk optics, such as beam splitter cubes, lenses, and mirrors, can more than double the module costs; and 2) the two laser diode chips double the semiconductor material costs. When these factors are accounted, the cost per Watt of the coupled power or multiplexed scheme is higher than a single laser coupled to the same fiber.
The present invention concerns a laser system utilizing multiplexed laser diodes. It uses a beam combiner that is integrated with the laser diodes on a single optical bench, and is thus, applicable to placement in a single pigtailed module. Specifically, multiple beams emitted from laser diodes are spatially merged using a birefringent material and then coupled into an optical fiber. The use of the birefringent material provides an feasible solution for generating the merged beam. The solution is most applicable, especially from a cost standpoint, to a configuration where the laser diodes are combined on a single chip with multiple, two, stripes or ridges.
In general, according to one aspect, the invention features a laser system. The system comprises an optical bench and a laser device, which is connected to the optical bench and emits multiple beams. A birefringent material, also connected to the optical bench, spatially merges the beams from the laser system. The merged beam is then coupled into an optical fiber.
In one embodiment, the laser device comprises a single chip. The chip, however, has multiple stripes or ridges, such that it is capable of generating the multiple beams. In the present embodiment, the laser diode chip has only two stripes to generate two beams.
One advantage associated with using semiconductor laser chips having multiple stripes arises from efficient utilization of the underlying semiconductor wafer material. Such dual or multi-stripe laser chips are less expensive per Watt of power generation capacity because the multi-stripe chip is not much larger in terms of wafer area than a single stripe chip because substantial wafer material area is lost to the scribe lanes required to separate chips during fabrication. With a multi-stripe laser chip, this lost material can be amortized over a larger number of the light-generating stripes.
In order for the birefringent material to perform the spatial merging function, the beams must have different polarizations with respect to each other when passing through the material, specifically linearly orthogonal polarization. Typically, when generated by one, multistripe chip, the beams will have similar polarizations with respect to the optical bench. As a result, provisions must typically be made for rotating at least some of the beams.
In the present embodiment, a polarization rotator is connected to the optical bench. It rotates a polarization of some of the multiple beams. As a result, the beams have different polarizations with respect to each other at the birefringent material.
In one implementation, the polarization rotator is a plate, such as a half-wave plate. In another implementation, the polarization rotator comprises a sub-wavelength period grating.
In the present embodiment, collimation optics is installed on the bench optically between the laser device and the birefringent material to counteract the beam divergence typical in semiconductor lasers.
Focusing optics is also preferably used, optically after the birefringent material, to focus the merged beam exiting from the birefringent material onto the end of the optical fiber pigtail.
In general, according to another aspect, the invention can also be characterized as a process for beam combination in a multi-beam laser system. As such, the invention comprises generating multiple beams from a multi-stripe laser chip. At least some of the beams have their polarizations subsequently rotated to enable spatial merging using a birefringent material. The merged beam is then coupled into an optical fiber.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.